In a digital oscilloscope, an input signal is first digitized by sampling the signal at discreet time intervals to obtain a digital value of the signal at each time interval. Each of these samples is then displayed on the CRT of the oscilloscope. There are three prior art methods commonly used for sampling of the signal: real time sampling, sequential repetitive sampling, and random repetitive sampling.
In real time sampling, the signal is digitized on the fly, in real time. There is a simple one to one correspondence between the samples and the times at which they were taken. That is, all the samples are taken during a single input waveform cycle. The advantage of real time sampling is that it takes all of its measurements over one cycle of the input, therefore, it is capable of sampling a single shot pulse with high throughput. The disadvantage of real time sampling is that it is impossible to do on very fast or high bandwidth signals, because the sample clock must be at least two times faster than the highest frequency component of the input signal.
In sequential sampling, one or more samples of the signal is digitized for each cycle of the input waveform. Therefore, the input signal must be repetitive, and the oscilloscope must be able to locate a triggering point within the repetitive waveform. With each successive triggering of the oscilloscope, new samples are taken. Each new sample point is delayed further from the trigger point than the previous sample, and the delay after each trigger is increased by a fixed amount from the delay of the previous trigger. This method guarantees that at least one sample is taken each trigger, therefore, it is faster at acquiring the waveform than random repetitive sampling and thus has higher throughput. The disadvantage of sequential sampling is that it can only sample positive time, that is, it can only take samples after the trigger.
Random repetitive sampling is similar to sequential sampling, except that the signal is constantly sampled and digitized at a rate determined by the oscilloscope's sampling clock, not by the input signal. After each sample is taken, the time relationship between the time of the sample and the trigger of the waveform is established. After a number of samples have been taken, the signal is reconstructed and displayed on the oscilloscope's display. The advantage of random repetitive sampling is that the waveform can be sampled before and after the trigger location, however, this method of sampling is slow in acquiring the signal over time ranges which are much shorter than the sample clock period and thus has low throughput. For acquisition time ranges less than the sample clock periods, a valid (inside time range) sample is not guaranteed with each trigger event. In fact, the probability of a sample landing inside the acquisition time range on any given trigger is equal to (acquisition time range) / (sample clock period).
Thus, there is a deficiency in the prior art sampling methods. Real time sampling has high throughput and is capable of sampling negative and positive time around the trigger event, however, because of circuit speed limitations it is only usable on low to medium bandwidth signals. Sequential sampling has high bandwidth and high throughput, however, it can only sample positive time after the trigger event. Random repetitive sampling has high bandwidth and can sample negative and positive time, however, it has low throughput, especially at fast sweep speeds.
Thus, there is the need in the art for a system that acquires data in both negative and positive time around the trigger event, is capable of acquiring very fast signals, and can rapidly acquire the signal for display to deliver high throughput. There is a further need in the art for a sampling system that has the high throughput of sequential sampling and the negative time sampling capability of random repetitive sampling. The present invention meets this and other needs.